Mechanical Engineering ETDs

Publication Date

Spring 5-13-2017

Abstract

Multilayer thin film composites, sometimes referred to as nanolaminates, have emerged as an important subset of materials with novel, and often tunable, properties such as high strength, high toughness, and resistance to wear or corrosion. Often fabricated using alternating layers of two or more materials, these multilayer thin film coatings are typically expensive and time intensive to fabricate and characterize and exhibit novel responses to nanomechanical testing such as plasticity during unloading. This thesis explores the nanoindentation response of hard/soft multilayer coatings through examination of the optical coating Al/SiC and similar coating Al/SiO2. Instrumented indentation was used to study single layer films of aluminum, silicon carbide, and silicon dioxide with thicknesses 40nm to 4μm. Results from individual and cyclic indentation load cycles provided insight into film mechanical properties. Additionally alternating 51 layers hard/soft multilayers on silicon and quartz substrates were studied with spherical and Berkovich indenters. These multilayer films were fabricated with bilayer thickness of 160nm but variable thickness ratio to achieve 25, 50, and 75% aluminum by volume. Further microstructural characterization is necessary to fully explain the indentation behavior, however an accurate prediction of indentation derived modulus for the nanolaminate based upon monolayer properties was found. Furthermore, cyclic indentation of the nanolaminates along with post-indentation TEM led to the conclusion that unloading plasticity was not occurring within the multilayer structure or the effect was not significantly altering the indentation response. Finite element simulations were created to model individual load cycles for each combination of indenter, thickness ratio, film material, and substrate using ABAQUS. Single layer and multilayer simulations exhibited plastic deformation increasing within the aluminum layers during the unloading phase of indentation for all cases. Further simulation was conducted focusing on the cyclic indentation of aluminum thin films and Al/SiC nanolaminates. It was concluded that the simulation adequately represented the single material film responses but were unable to predict the indentation-derived properties for the Al/SiC multilayer. Further investigation would benefit from knowledge of the ceramic microstructure and viscous properties.

Keywords

multilayer thin film nanolaminate nanoindentation aluminum

Degree Name

Mechanical Engineering

Level of Degree

Masters

Department Name

Mechanical Engineering

First Committee Member (Chair)

Mehran Tehrani

Second Committee Member

Yu-Lin Shen

Third Committee Member

William Mook

Document Type

Thesis

Language

English

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